Electrical heating element



une 29, 1943. B. HORSFIELD 2,323,089

ELECTRICAL HEATING ELEMENT Filed July 25, 1940 15a v Basil IrwLnTo Fig.5.

ATro mays said type.

Patented June 29, 1943 1 UNITED STATES PATENT OFFICE 2,323,039 ELECTRICAL HEATING ELEMENT Basil Horsfleld, Pittsburgh, Pa., asslgnor to Edwin L. Wiegand, Pittsburgh, Pa.

Application July 25, 1940, Serial No. 347,430

- March 30, 1915, the tool, of course, having a 17 Claims.

My invention relates to electrical heating elements of the type comprising a resistor disposed in and insulated from a metallic sheath. The principal object of my invention is to provide new and improved electrical heating elements 01 In the drawing accompanying and forming a part of this application, I have shown, for purposes of illustration, several forms of electrical heating elements of the aforesaid type, and in this drawing: a

Figure 1 is a plan view of an open face type heating,element of annular form,

Figure 2 is a plan view of the opposite side of the heating element shown in Figure 1,

Figure 3 is a section taken on the line 33 of Figure 1,

Figure 4 is a plan view, partly broken away, of another form of open face heating element, and

Figure 5 is a section taken on the line 55 of Figure 4.

The heating element illustrated in Figures 1,. 2, and 3 comprises a sheet metal sheath ID. The sheath I0 is of annular form and has disposed therein a resistor ii suitably insulated from the sheath ill by insulating material it. The resistor II is here shown as made of material of ribbon form, bent to sinuous form, but it will be apparent to those skilled in the art that it may be made of other forms of electric resistance material, wound or bent in other ways.

The sheath i0 is here shown as comprising a sheet metal annular channel i3 providing an annular plane wall I, the other side of the channel being left open, except-that, desirably, relatively small marginal portions ii of the sides of the channel are bent over the insulating material. The surface ii of the insulating material i2 is accordingly left exposed and, if desired, may be brought into direct contact with the part to be heated.

The heating element is provided with terminals in the form of studs l'l having any suitable form of heads IS, the shanks I! of the studs being desirably threaded to receive nuts 20. The wall H has apertures 2i through which the shanks l9 are disposed, and the shanks are suitably insulated as by mica washers 22, 23. The

ends of the resistor .H are suitably connected to the studs l1 respectively.

One way of constructing the heating element illustrated in Figures 1, 2, and 3 is as follows. The resistor II and studs II are supported on a suitable tool or form (not shown) analogous to thetool shownin the patent to Wiegand 1,133,347, 55

resistor-supporting part which is of such shape and size that it will fit into th channel II. The resistor I l, desirably arranged on and carried by the tool, is introduced into an open channel, such as I3 before the margins i6 are bent, the channel I! being partly filled to a predetermined level with insulating material which, whether it is mixed with a binder or not, is in a plastic, plastoid, or impressionable condition, and consequently the resistor becomes embedded in the insulating material. The tool is then withdrawn, leaving the resistor ll embedded in the insulating material: Additional insulating material is then introduced in the channel i3, over the embedded resistor, and the insulating material may then be compressed to a desired thickness. The margins liare bent over, and the element sublected to high pressure to compact the insulating material. The element may be subjected to various drying or baking steps, or both, at various intermediate stages of manufacture, and as a final treatment if desired.

Figures 4 and 5 show a heating element of the general type shown in Figures 1, 2, and 3, but of a shape adapted for use particularly for the heating of sole plates or flat irons. The element here shown comprises a sheath iOa of th general shape of a sole plate, having disposed therein a resistor Ila suitably insulated from the sheath by insulating material Ila. The resistor is here shown as made of helically wound wire, but may be of any other form. The sheath Illa comprises a sheet metal channel I30. providing a plane wall a, the other side of the channel being left open. Here too,margins i5a of the sides of the channel are desirably slightly bent over the insulating material I241. The element is provided with terminal studs ila similar to the terminal studs I! of the element 0t Figure 1.

The resistor Ha may be looped back and forth within th sheath Ilia any desired number of times, to provide any desired number of strands,

scribed in the patent to Wiegand 1,994,676, March 19, 1935. If desired a member of heat-insulating material 24 may be interposed between the insulated resistor lid and the wall Ila.

The body of insulating material embedding the resistor of the lieatingeiements must of course in Figures 4 and 5 may be be refractory, and it is desirable that as the chief component of the body of insulating material a material be selected that has suitable properties. as, for example, zircon (zirconium silicate), or any other refractory material having suitable electrical-insulating properties, desirably of mineral character, and desirably having good heatconducting properties. In the manufacture of the type of heating elements herein shown, a desired amount of clay or other bonding material may be mixed with the principal material so as to give a desired workability to the mass of insulating material during certain stages of the process of manufacture and, in general, to give the body of insulating material in the various stages of manufacture, and in the finished element, desired mechanical characteristics, but certain aspects of my invention are not limited to the use of bonding material.

I have found that the use of mullite as a component of the body of insulating material gives excellent results. Mullite is produced by concentrating the mineral kyanite to a high degree of mineral purity, this being then leached with sulphuric acid to remove the iron oxide in the crevices of the crystals. The material is then calcined at a temperature sufllcient to convert the suite crystal into a mullite crystal. There is no appreciable difference of composition but merely one of crystal form. Mullite may be obtained in a size -35 mesh and is a rather coarse granular aggregate containing however all of the fines produced during manufacture. The grains of mullite have a slightly porous surface.

I prefer to use mullite in combination with a granular heat-conducting electrical-insulating material which is of a hardness greater than mullite. For example, mullite is softer than zircon and, therefore, allows the zircon to become embedded more or less in the mullite under high pressure. This results in a hard solid cake. Preferably, the harder of the component materials is of a smaller mesh size than the softer material. It will be apparent that the principle of the embedding of a harder grain in a softer grain is applicable whether or not a bonding material is used in addition. Furthermore, because of this principle, if a bonding material is used lessis required.

By way of example of a mixture utilizing a bonding material, I may use approximately 18.1% mullite, 65.8% zircon, and 18.1% clay or kaolin. The zircon component of this mixture may to advantage comprise different portions of zircon of different mesh size, for example 42.8% unground zircon of -60 mesh, and 23.0% ground zircon of 200 mesh, the mullite being of mesh size.

Because of the slightly porous nature of the in which the results are very satisfactory both in mechanical workability in the assembly operations, and in the mechanical and electrical characteristics of the finished element. The material does not stick to dies used in the process of manufacture, and the resultant cake is of the desired hardness and avoids dlmcllltles hitherto experienced in the crumbling and cracking of the cake after baking. particularly on high temperature tests. The percentages of the ingredients are of course subject to variation, but I have given those approximate percentages which l have found to give the best results.

While I have given particular mixtures which are especially adapted for use in the open face type of electrical heating elements, it will be apparent to those skilled in the art that the principles of my invention maybe applied in other embodiments in other types of electrical heating elements.

When a bonding material such as clay is used, and the best insulating properties are desired, I prefer to use kaolin. Alkalis may be in the clay as a residue of decomposition of the granites, feldspars, or other minerals in the material from which the clay has been kaolinized. I prefer that the alkali content of the clay or kaolin be low,

preferably 0.25% or less. Preferably the calcium oxide content also should be low. Furthermore,

as clays contain mica, free silica, and other minerals, they often have to be washed and separated from these impurities. In the washing treatment of the clay no chemicals, basic or acid, should be used for agglomeration or dispersion as these are absorbed to some extent, and in clay for the present purpose they are detrimental impurities. After the washing treatment,'the material is dried and pulverized to a substantially uniform fineness of air-float size.

In general, in a body of insulating material embodying my invention and comprising a bonding material, such as clay, for example, the clay will of course be hardened somewhat by the baking or other heat treatment hereinbefore referred to, but the heat treating temperatures need rial.

From the foregoing it will be apparent to those skilled in the art that each of the disclosed embodiments of my invention provides a new and improved electrical heating element, and accordingly, each accomplishes the principal object of my invention. On the other hand, it also will be obvious to those skilled in the art that the disclosed embodiments of my invention may be variously changed and modified, or features thereof, singly or collectively, embodied in other combinations than those disclosed, without departing from the spirit of my invention, or sacrificing all of the advantages thereof, and that accordingly, the disclosure herein is illustrative only, and my invention is not limited thereto.

I claim:

1. An electrical heating element, comprising: a metallic sheath; a resistor in said sheath; and a body of insulating material in said sheath electrically insulating said resistor from said sheath, said body of material comprising approximately 16% mullite of approximately -35 mesh size, approximately 66% zircon of approximately --60 mesh size, and approximately 18% clay.

2. An electrical heating element, comprising: a metallic sheath a resistor in said sheath; and

a body of insulating material in said sheath elec trically insulating said resistor from said sheath,

said body of material comprising approximately 16% mulliteof approximately 35 mesh size, approximately 43% zircon of approximately 60 mesh size, approximately 23.0% zircon of approximately 200 mesh size, and approximately 18% clay.

3. An electrical heating element, comprising: a metallic sheath; a resistor in said sheath; and a body of insulating material in said sheath electrically insulating said resistor from said sheath, said body of material comprising a selected granular heat-conducting electrical-insulating refrac- 'tory material and mullite, mixed in the proportion of approximately one part mullite to four parts of said granular heat-conducting electrical-' insulating refractory material, said mullite comprising grains of larger mesh size than said selected material, and said selected material being harder than said mullite.

4. An electrical heating element, comprising: a metallic sheath a resistor in said sheath; and a body of insulating material in said sheath electrically insulating said resistor from said sheath, said body of material comprising zircon and mullite, mixed in the proportion of approximately one part mullite to four parts of zircon, said mullite comprising grains of larger mesh size than said zircon.

5. An electrical heating element, comprising:

a metallic sheath; a resistor in said sheath; and.

a body of insulating material in said sheath electrically insulating said resistor from said sheath, said body of material comprising approximately 16% mullite, approximately 66% zircon, and approximately 18% clay, said mullite comprising grains of larger mesh size than said zircon.

6. .An embedded-resistor electric heating element comprising a body of embedding material a major percentage of which is granular electrical-insulating refractory material approximately 7 to 30% of which is mullite.

7. An embedded-resistor electric heating element comprising a body of embedding material a major percentage of which is granular electrical-insulating refractory material the major percentage of which is appreciably harder than mullite and approximately 7 to 30% of which is mullite.

8. An embedded-resistor electric heating element comprising a body of embedding material a major percentage of which is granular elec-' trical-insulating refractory material the major percentage of which is zircon and approximately '7 to 30% of which is mullite.

9. An embedded-resistor electric heating element comprising a body of embedding material a major percentage of which is granular electrical-insulatingrefractory material approximately 20% of which is mullite.

10. An embedded-resistor electric heating element having a body of embedding material comprising a major percentage of granular electrical-insulating refractory material and a minor percentage of bonding'material, together with mullite in a percentage to appreciably decrease cracking under heat. I

11. An embedded-resistor electric heatingelement having a body of embedding material comprising a major percentage of granular elec-. trical-insulating refractory material appreciably harder than mullite, and a minor percentage of bonding material, together with mullite in a percentage to appreciably decrease cracking, under heat.

12. An embedded-resistor electric heating element having a body of embedding material comprising a major percentage of zircon and a minor percentage of bonding material, together with mullite in a percentage to appreciably decrease cracking under heat.

13. An embedded-resistor electric heating element having a body of embedding material comprising a major percentage of granular electrical-insulating refractory material and a minor percentage of bonding material, together with mullite of appreciably larger average grain size in a percentage to appreciably decrease cracking under heat.

14. For use in an embedded-resistor electric heating element, an embedding material comprising a major percentage of granular electrical-insulating refractory material and a minor percentage of bonding material, together with mullite in a percentage to appreciably decrease adhesion to pressing dies or the like.

15. For use in an embedded-resistor electric heating element, an embedding ,material comprising a major percentage of granular electrical-insulating refractory material appreciably harder than mullite, and a minor percentage of bonding material, together with mullite in a percentage to appreciably decrease adhesion to pressing dies or the like.

16. For use in an embedded-resistor electric heating element, an embedding material comprising a major percentage of zircon and a minor percentage of bonding material, together with mullite in a. percentage to appreciably decrease adhesion to pressing dies or the like.

17. I E'onuse-in an embedded-resistor electric heating element, an embedding material comprising a major percentage of granular electrical-insulating refractory material and a minor percentage of bonding material, together with mullite of appreciably larger average grain size in a percentage to appreciably decrease adhesion to pressing dies or the like.

BASIL HORSFIELD. 

